Lincoln's Inn Fields is the largest public square in London. Thank you Wikipedia. It was laid out in the 1630s under the initiative of the speculative
mission: impossible 5 Archives
Brendon tells me Mission: Impossible 5 actually won't be the first film to film inside the House of Parliament, as historical drama Suffragette already filmed there earlier this year, but Metro are reporting that Ethan Hunt and friends are indeed headed for the Commons.
According to the paper:
MI5 will feature numerous well-known locations in London but[...]